We propose to examine the biological response to treatment with anti-erbB2 antibodies and immunoliposomes in the treatment of breast cancer cells. The binding of lignad or antibody and the cell surface redistribution of erbB2 are the first step in signal transduction. We have only limited knowledge how these events are related to the final cellular response. The characterization of the role of cell surface interaction of erbB2 with itself (homoassociation) and with erbB3 or erbB4 will contribute to our understanding of how the sensitivity of breast tumor cell lines to anti-erbB2 antibody treatment is determined. We will define antibody and heregulin initiated changes in the cell surface interactions of erbB2. In an attemtp to correlate the efficiency of antibodies to halt the proliferation of breast tumor cells with molecular parameters, we plan to investigate the conformation of erbB2 on the surface of these cell lines. Molecular proximity can be accurately determined in living systems by the Forster-type resonance energy transfer method. This approach is applicable to flow cytometric and image microscopic experiments. The latter is able to resolve differences in the association state of proteins within single cells. We have developed a new method for image microscopic energy transfer measruements (using improved background subtraction and correction for spectral spillover). With this method not only can we detect subtle alterations in the homo- and heteroassociation of erbB2 within single cells, but we can also follow the time course of these changes and correlate these biochemical parameters with other cellular events, for example the internalization and tyrosime phosphorylation of the protein. Our work may contribute to better treatment of patients with breast cancer by pointing out the differences in the association state of erbB2 that may lead to resistance to antibody treatment.